Because someone who is great at mid champs can dominate with a mid, even if they're playing top, bot, (as a mage) etc. against abnormal matchups. Why? Because they know their strengths and weaknesses with whatever champ they're playing. If you're good at all roles you won't know the full extent of your champions prowess, you'll know a lot but not at all and that will leave you at a disadvantage against anyone that knows their champion/roles full capabilities and weaknesses. I play mid, I know my role as mid and I can win mid. Put me as an ADC? I'm the worst adc ever.
Originally Posted by The Mochi
Did I not say in my post that I understand why they DON'T do that?
Ok, I didn't think about lane swaps, but how about jungler and top being interchangable for example?
Because you play top and jungle COMPLETELY differently. Sure you will share a fair amount of champions between the roles, however the difference between jungle and top is vast.